Ball screw shaft, provided with vibration controlling means

ABSTRACT

A ball screw shaft, having a longitudinal cylindrical internal cavity, closed at its ends by means of inserted terminals or plugs, housing vibration controlling means in the form of a pair of plied cables fastened tensioned to the inserted terminals or plugs. One or more dissipater elements, having respective through holes for said pair of cables are provided positioned transversely free relative to an inner surface of said longitudinal cavity.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a ball screw shaft, providedwith vibration controlling means, of the kind having a longitudinalcylindrical internal cavity, closed at its ends by means of insertedterminals or plugs.

[0002] In ball screw devices of the kind with rotating nut screw, aswell as those in which the shaft rotates, the natural vibrationfrequency of the shaft remains low. Therefore, when the frequency ofsmall vibrations, applied to the shaft by the nut screw through itsvibration, are in area of the critical velocity of the shaft, suchresonance phenomena occur as to cause large vibrations in the screwshaft because of the great distance between the supports of the shaft.

[0003] Within the field of known solutions to the problem, it is foundthat the unexamined Japanese patent publication N. 6-11009 provides ascrew shaft bearing, in one its longitudinal cavity, a long stem servingas vibration controlling mass member, which comes in contact with theinner surface of said cavity when the shaft screw vibrates. Saidcollision limits the vibration of the screw shaft in its resonancepoint, which shaft is therefore able to operate at velocities exceedingits critical velocity. However, when the screw shaft is excited at lowfrequency, the screw shaft and the vibration controlling mass member donot always move relative to each other so as to touch each other, and inthe aforementioned patent no concrete means are indicated to allow thescrew shaft and the vibration controlling mass element to move in such away as to expect such a behaviour.

[0004] Subsequently, U.S. Pat. No. 5,613,400 has tackled the sameproblem of damping the transmission of vibrations from a nut screw to ascrew shaft to prevent it from resonating and allow an operation with along travel and high speed. This patent discloses a ball screw devicehaving a screw shaft including a longitudinal internal cavity with alongitudinal vibration controlling mass member within it, held bysupport bushings, and a damping element interposed between the vibrationcontrolling mass member and the inner surface of the longitudinal cavityof the screw shaft to prevent contact between the vibration controllingmass member and the longitudinal cavity. According to the aforementionedpatent, the vibration controlling mass member is constituted by a singlerigid steel hold ring fastened to the ends of the screw shaft.

[0005] Although said steel hold ring, as a vibration controlling means,is provided associated to various damping and supporting means indifferent embodiments, nonetheless it cannot provide a satisfactorydamping effect in a low frequency area and limit vibrations in a widerange of frequencies in correspondence with the natural frequency of thescrew shaft that varies with the position of the nut screw.

SUMMARY OF THE INVENTION

[0006] The present invention is therefore aimed at solving the problemstated above, overcoming the limits and drawbacks of the prior art.

[0007] Thus, the main aim of the invention is to provide vibrationcontrolling means that are able by themselves to allow a drasticreduction of vibrations, preventing them from reaching a condition ofresonance.

[0008] Another aim of the invention is to provide dissipater means madeof materials with such intrinsic characteristics as to transform thevibration energy into other forms of energy, especially into thermalenergy.

[0009] The vibration energy content to be transformed is in any caselow, so that it does not cause the temperature of the screw shaft torise.

[0010] Thus, the invention provides a ball screw shaft, provided withvibration controlling means, of the kind having a longitudinalcylindrical internal cavity, closed by means of inserted terminals orplugs at the ends of the screw shaft and housing vibration controllingmeans, which, from a general point of view, is characterized in thatsaid vibration controlling means are constituted by a pair of steelplied cables fastened to said inserted terminals or plugs; one or moredissipater elements, having respective through holes destined to receivein pass-through manner, in predetermined position and distance, saidpair of cables, being provided transversely free relative to the innersurface of said longitudinal cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Further characteristics and advantages of the present inventionshall become more readily apparent from the detailed description thatfollows of a preferred embodiment, illustrated purely by way of nonlimiting indication, in the accompanying drawings in which:

[0012]FIG. 1 shows a longitudinal section of a screw shaft according tothe present invention;

[0013]FIG. 2 shows an enlarged scale longitudinal section of an endportion of the screw shaft of FIG. 1;

[0014]FIG. 3 shows a schematic axonometric view of a dissipater element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] With reference to FIG. 1, a ball screw shaft is globallyindicated with the reference number 1, and a nut screw advancing thereonis indicated with 2. The screw groove of the shaft is indicated onlypartially with the reference 3, although it should be understood that itis provided over the whole length of the screw shaft 1. The nut screw 2is also shown only partially and it is not described, as it is a whollyconventional device. Purely by way of example, the screw shaft issupported by end stands generally indicated with the number 4, butobviously they can be replaced with other support means depending on theapplication. The shaft 1 has a longitudinal cylindrical internal cavity5, which is closed at its ends by means of inserted terminals or plugsgenerically indicated with the number 6, hereinafter for the sake ofsimplicity referred to only as terminals. Inside the longitudinal cavity5, according to the invention, vibration controlling means areconstituted by a pair of steel plied cables, indicated with thereference number 7 and, respectively, 8.

[0016] The cables 7 and 8 are positioned parallel, held tensioned at theends by effect of a pre-tensioning obtained when mounting the terminals6. The cables 7 and 8 are fastened to the terminals 6 in predetermineddistanced positions.

[0017] The cables are mutually distanced with one or more dissipaterelements 9, preferably in the form of disks of elastomeric material.They are destined to receive in pass-through fashion through respectiveholes 10 and 11 (FIG. 3), in predetermined position and distance, thepair of cables 7 and 8, being positioned transversely relative to theinner surface of the longitudinal cavity 5 (FIG. 1).

[0018] The cables 7 and 8 preferably have different diameters toguarantee the onset of mutually different vibratory phenomena, able oactivate the dissipaters made of elastomeric material.

[0019] The cables themselves, because of their plied configuration, aresystems with high internal damping.

[0020] Additional, the presence of oil, particulate or other dissipatingmaterials can be provided, to fill the free cavity between the cablesand the inner wall of the longitudinal cavity 5 of the shaft and therebyenhance internal damping.

[0021] To demonstrate the improvements obtained with respect to theprior art, comparison tests were conducted on known screw shafts and ascrew shaft according to the present invention.

[0022] For the tests, a test bench was selected in which the screw shaftis locked in place at the end with respective hydraulic vices. Duringeach test the screw shaft was subjected to impact stress using aninstrumented hammer PCB 086B04 with rubber tip. The response of thescrew shaft was measured with a piezoelectric accelerometer whosenatural frequency is equal to 51 kHz. The piezoelectric accelerometerwas positioned at the centre of the screw, where acceleration isgreatest. Both the signal of the instrumented hammer (force signal) andthat of the accelerometer (acceleration and displacement signal) weresampled. To improve signal-to-noise ratio, multiple-sample averages wereobtained for each measurement point.

[0023] Analysing the results of the tests performed on the screw shaftaccording to the present invention, compared to the results on the priorart screw shaft, a considerable increase was noted in the internaldamping of the system: displacement amplitude rapidly drops to nil about0.9 seconds after the hammer blow.

[0024] Naturally, the invention thus conceived can be subject tonumerous modifications and variations, without thereby departing fromthe scope of the inventive concept.

What is claimed:
 1. Ball screw shaft, provided with vibrationcontrolling means, of the kind having a longitudinal cylindricalinternal cavity, closed by means of inserted terminals or plugs at theends of the screw shaft and housing vibration controlling means, whereinsaid vibration controlling means are constituted by a pair of steelplied cables fastened tensioned to said inserted terminals or plugs; oneor more dissipater elements, having respective through holes destined toreceive in pass-through fashion, in predetermined position and distance,said pair of cables, being provided positioned transversely freerelative to the inner surface of said longitudinal cavity.
 2. Screwshaft as claimed in claim 1, wherein said cables have equal diameter. 3.Screw shaft as claimed in claim 1, wherein said cables have differentdiameters.
 4. Screw shaft as claimed in claim 1, wherein said dissipaterelements are disk shaped.
 5. Screw shaft as claimed in claim 1, whereinsaid dissipater elements are made of elastomeric material.
 6. Screwshaft as claimed in claim 1, wherein said longitudinal cavity, freebetween the cables and the inner wall of the shaft, is filled withdissipating material able to enhance internal vibration damping.